"We figured out at NASA how to do it in about 10 years for $40 billion," said Charles Miller, who recently left his position as NASA Headquarters' senior adviser for commercial space and is now president of NextGen Space. "The question is, would Mitt Romney fire me for a proposal to return to the moon for $40 billion?"

For a few years, NASA was following a plan to return to the moon by 2020 for $104 billion, through the Constellation program set up under President George W. Bush. But Constellation was canceled by President Barack Obama, and the space agency currently is gearing up for an effort to put astronauts on a near-Earth asteroid by the mid-2020s.

The moon-shot cancellation was in line with an independent panel's conclusion that the plan was "not viable," considering the realities of the federal budget. But that panel was working under the assumption that a whole new deep-space infrastructure would have to be developed, including a heavy-lift vehicle then known as the Ares 5. That assumption was carried over into the post-Constellation plan, in the form of a heavy-lift Space Launch System that would cost $35 billion over the next decade or so. Billions more would have to be spent preparing for trips beyond Earth orbit — to an asteroid, to the moon, to Mars or other destinations.

Plan B for outer spaceMiller and his colleagues on a NASA task force drew up an alternative plan, which they said would provide a less expensive and faster path to deep-space exploration. Rather than building an entirely new type of heavy-lift rocket, NASA would use a series of tried-and-true rockets — perhaps including the U.S. commercial Atlas, Delta and Falcon rockets as well as Europe's Ariane, Japan's H2 and Russia's Soyuz and Zenit rockets — to deliver propellant to an orbiting fuel depot.

After a series of low-cost fuel delivery flights, the high-value components for trips to the moon would be sent up and assembled in orbit. Once the lunar transfer vehicle was ready to go, the astronauts would climb aboard, head out of orbit for the moon, conduct their mission and return.

NASA would still have to develop a lunar lander, as well as the Orion deep-space capsule it's currently working on, and perhaps a habitat module as well. But it wouldn't have to build the heavy-lifter.

A preliminary version of the plan was leaked to the SpaceRef website last October, amid calls from Rep. Dana Rohrabacher, R-Calif., for the report's release. At the time, the report suggested that missions to the moon could begin in 2024, but Miller told me that he challenged his team to optimize their cost and timeframe estimates. "They went from landing on the moon in 2024 to 2021," he said, at an average cost of $4 billion per year for 10 years. Such funding levels would be in line with NASA's current budget, with adjustments for inflation in the latter five years, he said.

NBC News space analyst James Oberg talks about whether Newt Gingrich's vision of a colony on the moon contains any benefits, and what the price tag might look like.

Miller said the plan could conceivably be revised to reduce the time frame even further, from 10 to eight years. "It's ready when our national leadership decides it wants something more affordable," he told me. "I consider it to be Plan B."

NASA has not released the current version of the plan, but the agency's top executives have not been as bullish as Miller is about Plan B. During a congressional hearing last summer, NASA Administrator Charles Bolden said the alternatives to building a heavy-lift rocket were "not as economical, nor as reliable."

Miller contends that the plan didn't get a proper "apples-to-apples" comparison from NASA's top executives or from the Human Exploration Framework Team, which drew up NASA's Plan A.

Reality checkIt may well be technically possible to send astronauts back to the moon by 2020 — but even if NASA successfully implemented the Plan B outlined by Miller, there wouldn't be a full-fledged moon colony by that time. Then there's the bigger question of whether it's worth spending tens of billions of dollars to put astronauts back on the moon, even if the experts agree it's possible to do it within a $40 billion budget.

"That's what you hire presidents to decide," Miller said.

Obama decided years ago that it would be better to go to a new destination in deep space, such as a near-Earth asteroid or the moons of Mars, rather than returning to the moon. "We've been there before," Obama said when he announced his space goals in 2009.

It's possible that Gingrich's pledge to build a moon base by 2020 has hurt him in the polls — even in Florida, where the aerospace industry has suffered a heavy blow due to last year's retirement of the space shuttle fleet. On the eve of Florida's primary, surveys suggest that Gingrich is lagging by double digits behind Romney, who has been far less specific about his space aspirations. In effect, Romney wants to conduct another round of soul-searching about NASA's vision, retracing the process that Obama and his aides went through three years ago.

For now, NASA's big-ticket priorities in human spaceflight are to continue developing the Space Launch System and the Orion multipurpose crew vehicle, while commercializing operations to send supplies and astronauts to the International Space Station. The Space Launch System in particular has strong support in Congress — so much so that critics have dubbed it the "Senate Launch System." Any effort to change course at this point would probably run into significant opposition — unless the SLS project became totally unworkable and/or unaffordable.

In that case, Plan B ... or Plan C, or D ... might well get another look, regardless of who's in the White House.

Update for 7:50 p.m. ET: Over the past week, there's been a lot of debate over Gingrich's moon-base pledge, and over the justification for spending anything at all on space exploration. I've tried to step around the questions surrounding the rationales for spaceflight in this item — but Wayne Hale, who used to head up NASA's space shuttle program, provides a provocative perspective today in a posting to his blog, titled "What Would Rick and Gus and Dick Want?" The title is a reference to the anniversaries of the Columbia tragedy (helmed by Rick Husband), the Apollo 1 fire (with Gus Grissom as commander) and the Challenger explosion (commanded by Dick Scobee). Here's some of what Hale says:

"It is impossible to build a business plan on exploration of the unknown; some decisions aren’t amenable to the quarterly profit and loss statement. Seward’s folly, Jefferson’s gamble, Teddy’s canal – they were all the butt of jokes and sarcasm. Yet, America, the land of opportunity, was not built by skeptics. America was built by people who were willing to risk everything on a dimly perceived future. Facing the unknown frontier changed Americans and made us what we are. We would be a lesser people if our great-grandparents had not chosen those challenges. The cost was high and many did not live to see the results of their gamble. But as a nation we continued on and became great.

"Now where is our frontier? Making corporate profits on Wall Street by moving money around? Now what will inspire our children? Playing video games that are made in overseas sweatshops?

"You know better than that. Without the challenge of a frontier, stagnation, mediocrity and decline is our guaranteed future."